In a large pot, heat the butter on medium-high heat. Add your chopped onions and fry for about 1-2 minutes. Sprinkle the starch over the onion and stir for about 30 seconds. Add your cold broth and stir immediately! Add your potato cubes to your broth, and cook them until they are tender. Add your 1/2 pound spinach leaves and keep cooking on medium-high heat for about 10 min. With a hand blender, purée the whole soup until you have no bits left over. Add the yogurt,lemon juice, fresh mint and dry mint, mix well until all is well combined. Add the chopped spinach leaves and pressed garlic. Season a good amount with salt an pepper to taste. Serve Hot!